1. Field of the Invention
This invention relates to a process of making the low sensitivity, high energy density solid oxidizer 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.05,903,11]-dodecane, also known as xe2x80x9cTEXxe2x80x9d. The process provides several advantages over known processes, including faster reaction times with excellent yields and product purity.
2. Description of the Related Art
A synthesis route for preparing TEX is disclosed in U.S. Pat. No. 5,498,711, the complete disclosure of which is incorporated herein by reference. According to the ""711 patent, TEX is synthesized by reacting 1,4-diformyl-2,3,5,6-tetrahydroxypiperazine and derivatives thereof with a strong acid and a nitrate source at temperatures greater than ambient temperature, such as temperatures in a range of 50xc2x0 C. to 70xc2x0 C. The strong acid and nitrate source of preference are sulfuric acid and nitric acid, respectively. The reaction is exothermic and is allowed to continue for two to three hours. The mixture is then poured onto ice, and a solid precipitate is isolated and washed to give a mixture which contains the TEX. The reaction is shown below: 
Purification can then be accomplished by heating the reaction product in nitric acid, washing with methanol, and/or washing with a base to neutralize excess acid. The pure product may be obtained by recrystallization according to standard procedures.
The synthesis route reported in the ""711 patent produces TEX in yields and purities that constitute improvements over the known art. However, as is evident from the relatively long reaction times of 2 to 3 hours and the use of NOx scavengers, the ""711 patent teaches not terminating the reaction until after TEX is precipitated out and NOx by-product gases are generated. The generation of NOx gases, such as NO2, is an autocatalyzing reaction that becomes rapid within a relatively brief period of time, with the actual period depending on various factors, such as piperazine derivative concentration and make-up of the acid bath. Rapid generation of NOx causes fume off of reactants and product, which lowers product yield and makes the process less conducive to applications in large-scale TEX production operations.
It is, therefore, an object of this invention to address a need in the art by providing a method by which 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.05,903,11]-dodecane (herein referred to as TEX) can be synthesized at a sufficiently high purity and yield to permit its large scale production in an economically feasible manner.
In accordance with the objects of this invention, these and other objects are accomplished by the provision of process in which TEX is prepared by reacting at least one suitable hexa-substituted piperazine derivative (preferably 1,4-diformyl-2,3,5,6-tetrahydroxypiperazine and/or a derivative thereof) in a medium comprising at least one nitrate source and optionally at least one strong acid at temperatures equal to or greater than ambient temperature, with the reaction temperature preferably being in the range of from about 50xc2x0 C. to about 80xc2x0 C. During the initial stages of the reaction, which are highly exothermic, the temperature of the medium is maintained in a range of from about 50xc2x0 C. to about 80xc2x0 C. using cooling techniques. Subsequently, the initial exotherm ceases or substantially abates, at which point cooling can be terminated while still maintaining the reaction temperature in the range of from about 50xc2x0 C. to about 80xc2x0 C. Eventually, NOx generation commences via an exothermic autocatalytic reaction. Although the inventors do not wish to be bound by any theory, it is believed that the autocatalytic reaction is dependent upon there being a predetermined amount of water present in the medium. The reaction is stopped by cooling the medium to a temperature sufficiently low either to prevent onset of the NOx autocatalytic stage or, if the NOx autocatalytic stage has already commenced prior to the final cooling step, to terminate the autocatalytic generation of NOx. Preferably, the autocatalytic generation of NOx is terminated before a sufficient exotherm has been released to permit the medium (while not being cooled) to be raised in temperature by more than 5xc2x0 C. The reaction product is precipitated in a conventional manner, preferably by cooling at room temperature, and purified to yield TEX.
Advantageously, because the heat source is removed prior to or just after the formation of NOx, the rapid release and fume off of NO2 is avoided. Furthermore, unlike many conventional processes, a NOx scavenger, such as urea, may be omitted from the reaction to reduce the likelihood of by-product forming reactions. Moreover, unlike conventional processes, in a preferred embodiment the present invention is conducted in a medium which is free or substantially free of a strong acid other than nitric acid. Counter-intuitively, it has been found that the elimination of a strong acid, such as sulfuric acid, increases the TEX formation rate.
The present process can be conducted on a large manufacturing scale in which the exothermic reaction process is controllable, while directly yielding greater amounts of TEX in a high purity sufficient for use in formulating explosive compositions. These advantages are obtainable without requiring the heretofore extensive further purification or recrystallization steps.
Other objects, aspects and advantages of the invention will be apparent to those skilled in the art upon reading the specification and appended claims, which explain the principles of this invention.
In the present process, TEX is prepared by addition of a predried mixture of a hexa-substituted piperazine derivative to a heated acid medium comprising at least one nitrate source and, optionally, at least one strong acid.
Hexa-substituted piperazine derivatives suitable for use in the present process are represented by the following general formula (1): 
wherein xe2x80x94OR is a good leaving group and R is H, Rxe2x80x3, xe2x80x94CRxe2x80x3O, xe2x80x94CORxe2x80x3, xe2x80x94COORxe2x80x3, xe2x80x94SO3Rxe2x80x3, xe2x80x94NO, xe2x80x94NO2, acetal (including aliphatic (e.g., formal), cycloaliphatic (e.g., cyclohexanal), and branched acetals (e.g., dimethylketal)), and cycloacetals; Rxe2x80x2 is a nitrolyzable group such as xe2x80x94CRxe2x80x3O, xe2x80x94CORxe2x80x3, xe2x80x94SO2Rxe2x80x3, xe2x80x94SO3M, xe2x80x94NO2, xe2x80x94NO, xe2x80x94COORxe2x80x3, t-butyl, cyclohexyl, and isopropyl; M is an alkali metal, preferably lithium, sodium, or potassium; Rxe2x80x3 is H, C1 to C10 alkyl, branched alkyl, cycloalkyl, and aryl (such as phenyl and substituted phenyl) and monocyclic heterocyclic moieties, and wherein each R, Rxe2x80x2, or Rxe2x80x3 can independently be the same or different. As used herein, phenyl substituents include, but are not limited to, C1 to C10 alkyl, branched alkyl, halogen, nitro, amino, substituted amino, alkoxy, acyl, and carbonyl containing moieties such as carboxyl, ester, ketone, etc. Exemplary, suitable monocyclic heterocyclic moieties contain one or more heteroatoms such as nitrogen, sulfur, and/or oxygen (e.g., triazinethiophenefuran). Representative hexa-substituted piperazine derivatives include, for example,1,4-bis(methylsulphonyl)-2,3,5,6-tetrahydroxypiperazine, disodium-2,3,5,6-tetrahydroxypiperazine-1,4-disulphonate, 1,4-diformyl-2,3,5,6-tetraacetoxypiperazine, and, 1,4-diformyl-2,3,5,6-tetrahydroxypiperazine (THDFP). Of the above hexa-substituted piperazine derivatives, THDFP is the preferred starting material. THDFP is illustrated below by the following formula (2): 
Typical hexa-substituted piperazine derivatives which may be used to synthesize TEX can be prepared by reacting glyoxal with an amide, sulfonamide, or sulfonate salt in known methods. Other hexa-substituted piperazine derivatives which may be used in the present invention are reported in Currie, A. C., et al., xe2x80x9cBase-catalysed Reactions of Glyoxal. Part I. 1,4-Diformyl- and 1,4-Bismethylsulphonyl-Derivatives of 2,3,5,6-tetrahydroxypiperazines,xe2x80x9d Journal of the Chemical Society (Sect. C), pp. 491-496 (1967) and Dinwoodie, A. H. et al, xe2x80x9cBase-catalysed Reactions of Glyoxal. Part II. 2,3,5,6-Tetrahydroxypiperazine-1,4-disulphonic Acid Derivatives,xe2x80x9d Journal of the Chemical Society (Sect. C), pp. 496-497 (1967).
As used herein, diacyltetraoxypiperazine derivatives also include TEX intermediate products, such as tetraoxadiazaisowurtzitane derivatives which may be prepared from diacyltetraoxypiperazine derivatives.
By preference, the present process also includes several features to reduce the amount of water present in the reaction mixture. The presence of water in the reaction mixture can affect the overall reaction by (a) increasing the NOx produced which increases reaction instability, and (b) decreasing the nitrating strength of the acid which, in turn, decreases yield of TEX.
The hexa-substituted piperazine derivative can be can be subject to a pre-drying treatment prior to addition to the acid medium to obtain, by preference, a finely ground composition. Suitable drying conditions include, for example, overnight drying or drying for about 24 hours under vacuum, typically about 10 mm Hg, at about 50xc2x0 C. to about 60xc2x0 C. The upper temperature is limited by the decomposition temperature of the reactants, although the upper temperature can be on the order of about 150xc2x0 C. This predrying process removes residual water from these materials. As stated above, the predrying treatment results in an increased TEX yield.
The reaction vessel is optionally purged with an inert gas, and the reaction is preferably conducted in a range of from about 50xc2x0 C. to about 80xc2x0 C., more preferably about 50xc2x0 C. to about 70xc2x0 C. If the reaction is conducted at too low a temperature, for example, below about 50xc2x0 C., then the formation of undesired side products is increased and yield of TEX is decreased. In contrast, too high a reaction temperature, for example, above about 80xc2x0 C., results in increased reaction instability and likelihood of fume-off, increased NOx production, and lower TEX yield.
The medium to which the hexa-substituted piperazine derivative starting material is added preferably is preheated to a temperature of between about 55-60xc2x0 C. Suitably the lower temperature of the medium does not fall below about 55xc2x0 C. Preferably, the medium temperature is regulated to a maximum of approximately 80xc2x0 C., although more preferably it is approximately 70xc2x0 C.
Suitable nitronium ion sources include nitric acid and/or ammonium nitrate. Optionally, strong acids may also be used with the nitronium ion source. Representative strong acids include inorganic acids, such as sulfuric acid, oleum, nitric acid, or hydrohalo acids, such as, hydrochloric acid. Organic acids and anhydrides thereof, such as, trifluoroacetic acid (TFA), and trifluoroacetic anhydride (TFAA), are also suitable for use in the present invention. Preferably, the only strong acid present in the acid medium is nitric acid, and more preferably 100% concentration nitric acid. Where a strong acid other than nitric acid is used, the volumetric ratio of nitric acid to the combination of the other strong acids should be at least about 5:1, preferably about 10:1, and more preferably at least about 20:1. The ratio of nitrate source and strong acid (ml) to grams of hexa-substituted piperazine derivative starting material(s) preferably is at most about 8:1, and more preferably is in a range of from about 4:1 to about 5:1.
The present invention is preferably conducted in an acid medium which is free or substantially free (i.e., not more than 10 vol %) of a strong acid (other than nitric acid). Intuitively, it would seem that the rate of reaction could be increased by increasing the strong acid, e.g., sulfuric acid, concentration in the acid medium, since an increase in sulfuric acid concentration generates a corresponding increase in nitronium ion activity. However, the present inventors discovered, to their surprise, that high concentrations of strong acids, such as sulfuric acid, decrease the rate of TEX formation by promoting foaming during exothermic stages of the reaction.
An inert co-solvent may also be added to the acid medium prior to heating. The inert co-solvent acts as a thermal diluent and heat transfer agent by absorbing the heat generated by the exothermic reaction, boiling, and transferring the heat to a reflux condenser. The inert co-solvent further reduces the intensity of the exotherm and the probability of an uncontrollable reaction. Representative inert co-solvents include 1,2-dichloroethane, methylene chloride, and tetramethylene sulfone (sulfolane).
The hexa-substituted piperazine derivative can be added to the acid medium, or vice versa, at once or in a stepwise or continuous manner. The duration of each stage will depend on several factors, including reaction temperature and acid ratios. Although this invention is not thereby limited, the initial exotherm generally lasts for approximately 2-8 minutes, followed by a substantially non-exothermic stage of approximately 2-15 minutes, followed by the autocatalytic NOx stage.
The reaction product is precipitated by cooling, such as in an ice bath, followed by filtering and purifying. Currently preferred purification techniques include heating the reaction product in nitric acid, washing with methanol, and/or washing with a base to neutralize excess acid.
The pure product may be obtained by suitable separation techniques, such as crystallization or recrystallization techniques known to those skilled in the art. Typical crystallization solvents which may be used include acetonitrile, acetone, butyrolacetone, nitric acid, ethyl acetate, pyridine, DMSO, and DMF.
Typically, TEX yields are greater than 20% by weight based on the amount of piperazine starting material, and the purity is typically 98% or greater based on proton NMR analysis.
The TEX as obtained can be utilized in explosive compositions without the need for further purification or recrystallization steps. The use of TEX in explosive compositions is discussed in greater detail in U.S. Pat. No. 5,529,649, the complete disclosure of which is incorporated herein by reference. TEX may be used alone or in combination with conventional or novel solid explosive ingredients as the basis for formulating very high performance insensitive explosive compositions, such as taught in U.S. Pat. No. 5,587,553, the complete disclosure of which is incorporated herein by reference. For example, TEX may be used in combination with at least one binder, metal, and oxidizer, and optionally other explosive compounds to prepare low cost, castable explosives. Typical formulations may contain from about 5% to about 90% TEX, preferably from about 30% to about 90% TEX; from about 10% to about 30% binder; from about 0% to about 50% oxidizer; and from about 0% to about 30% reactive metal.
Representative inert polymeric binders include HTPB (hydroxy-terminated polybutadiene), PBAN (butadiene-acrylonitrile-acrylic acid terpolymer), PPG (polypropylene glycol), PEG (polyethylene glycol), polyesters, polyacrylates, polymethacrylates, CAB (cellulose acetate butyrate), or mixtures thereof. Representative energetic polymeric binders include PGN (polyglycidyl nitrate), poly-NMMO (nitratomethyl-methyloxetane), GAP (polyglycidyl azide), 9DT-NIDA (diethyleneglycol-triethyleneglycol-nitraminodiacetic acid terpolymer), poly-BAMO (poly(bisazidomethyloxetane)), poly-AMMO (poly(azidomethyl-methyloxetane)), poly-NAMMO (poly(nitraminomethyl-methyloxetane)), copoly-BAMO/NMMO, BAMO/AMMO, nitrocellulose, or mixtures thereof. The binder can optionally be halogenated, such as fluorinated ethylene propylene copolymer, chlorotrifluoroethylene and vinylidene fluoride copolymer, polyvinylidene fluoride, polydifluorochloroethylene, fluorinated polyethers, PVC, polytetrafluoroethylene, or mixtures thereof.
Representative oxidizers include AP (ammonium perchlorate), AN (ammonium nitrate), HAN (hydroxylammonium nitrate), AND (ammonium dinitramide), or mixtures thereof.
Representative reactive metals include aluminum, magnesium, boron, titanium, zirconium, or mixtures thereof.
Other explosives that can be used in combination with TEX include RDX (1,3,5-trinitro-1,3,5-triaza-cyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraaza-cycloocatane), NTO (3-nitro-1,2,4-triazol-5-one), NQ (nitroguanidine), HNIW (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,903,11]dodecane), TATB (1,3,5-triamino-2,4,6-trinitrobenzene), and DADNE (1,1-diamino-2,2-dinitro ethane).
TEX and a small amount of binder may also be used to prepare high solids ( greater than 90% TEX) pressable or extrudable explosives. The pressable or extrudable explosives have a high solids content and contain up to about 98.5% TEX, preferably from 50% to 98.5% TEX, and most preferably from 80% to 98.5% TEX, or a combination of TEX and other explosive. The pressable or extrudable explosives can also contain inert and/or energetic plasticizers. Representative inert plasticizers include DOA (dioctyladipate), IDP (isodecylperlargonate), DOP (dioctylphthalate), DOM (dioctylmaleate), DBP (dibutylphthalate), oleyl nitrile, or mixtures thereof. Representative energetic plasticizers include BDNPF/BDNPA (bis(2,2-dinitropropyl)acetal/bis(2,2-dinitropropyl)formal), TMETN (trimethylolethanetrinitrate), TEGDN (triethyleneglycoldinitrate), DEGDN (diethyleneglycol-dinitrate), NG (nitroglycerine), BTTN (butanetrioltrinitrate), alkyl NENA""s (nitratoethylnitramine), or mixtures thereof.
Melt cast explosives may be prepared by combining TEX with an energetic or inert material having a relatively low melt temperature ( less than 120xc2x0 C.). Representative meltable energetic materials include TNT (2,4,6-trinitrotouene) and TNAZ (1,3,3-trinitroazetidine). Other meltable energetic materials which may be used include AN/NQ eutectic or alkylammonium nitrate salts. Inert meltable materials such as polyethylene and hydrocarbon wax may also be used. The melt cast explosives may also contain a metal, oxidizer and other nitramine.
The following examples are offered to further illustrate the synthesis methods of the present invention. These examples are intended to be exemplary and should not be viewed as a limitation on the claims.
In the following examples, unless otherwise specified nitric acid (90% ACS Grade) and sulfuric acid (98%, ACS plus) from Fischer, nitric acid (100%) from Fluka, and THDFP from Parish Chemical Company were used without purification.